It depends what you like to see. I'm guessing your route's something like St Malo - Rennes - Nantes - La Rochelle. But each of these places has something to offer, and places in between too.
St Malo has a great, if slightly touristy walled town dating back to the 1950's (OK dating back a whole load further but extensively and carefully restored after the Allied bombings of WW2). Nearby there's Cancale, and a little further over Mont St. Michel.
Rennes has a medieval quarter, some imposing public buildings like the Parliament, the Opera and some great public parks.
Nantes is huge, but fascinating for its architecture, it's naval history, and it's full of great restaurants.
And then there's all the bits in between too, watching as the scenery changes from Brittany, which can be quite British looking, with granite houses and slate roofs, then the sudden change south of Nantes where you start seeing flatter terracota tiled roofs, and vines growing and signs to places which you thought were only found on wine labels.
I'm sorry, that's not much help really you could spend days in each town, just wandering about, I mean I've lived in Brittany for 11 years and I've still got year's more exploring to do.
One thing though, For the St Malo to Nantes section there's no Autoroutes (like motorways where you pay a toll and can do 130km/h) so even the main roads can be and are very scenic.
Best thing I reckon for you as you've got a packed itinerary would be to drive, and when you feel like a break pull off the main road and find a little village, stop at a café, get ice-creams and have a wander to stretch your legs, I always reckon France is best take at a leisurely pace and if you can't do that then join up some relaxed wandering with some high speed highway driving, at least that way you get a taste of la vie francaise.
Just don't eat in motorway servicestations and don't restrict yourself to McDo, even if you can get a
with your Royale with Cheese.